The infection makes the rats less fearful of noveltyImage by Manuel Berdoy

By BBC News Online's Matt McGrath

A parasitic infection in some rats alters their natural behaviour and makes them easy prey for cats, research shows.

Toxoplasma gondii is found in about 35% of ratsImage by Dr Boris Striepen, Georgia University

Scientists at Oxford University in the UK say the parasite Toxoplasma gondii is found in about 35% of rats but prefers to live in cats.

It ensures its return to its favourite host by affecting the brains of the normally cautious rats, making them outgoing and active and an easy meal for a hungry cat.

Dr Manuel Berdoy, and colleagues, report their work in the Proceedings of the Royal Society.

Dr Joanne Webster: We are likely to find more evidence of the parasite's impact on children

One of the team, Dr Joanne Webster, a lecturer in infectious disease at Oxford,
told BBC News Online: "The infection makes the rats less fearful of novelty.

"Rats can usually detect subtle changes in their environment. It makes them very hard to trap or poison but this parasite overrides the innate response - they almost taunt the cats in a sense."

In rats, the infection is usually contracted from eating cat faeces. The parasite then works itself into every organ in the rat's body especially the brain where it forms dormant cysts.

There it remains viable for the lifetime of the rat, waiting for a cat to come along and eat the rodent so that it can spring to life again.

Impact on children

Dr Webster has been studying the effects of this parasite for years.

This parasite overrides the innate response - they almost taunt the cats in a sense.

Dr Joanne Webster

While the mechanism by which it works on the brains of rats is uncertain, she has no doubt about its powerful impact.

"None of the other parasites I've ever looked at have had any of these effects upon behaviour," she said.

The parasite is also widespread in human brains, but does not cause a problem unless the immune system is compromised.

"If we become immuno-suppressed with Aids or chemotherapy, the parasites reactivate in us. Basically, this is what causes the madness at the terminal stage of Aids," Dr Webster said.

The parasite is picked up in cat faecesImage by Manuel Berdoy

"It's actually the Toxoplasma making great big holes in your brain."

She also says we are likely to find more evidence of the parasite's impact on children.

"There is some initial research that has found hyperactivity and low IQ in children with high Toxoplasma levels.

"I don't think we can be that dismissive of such a prevalent parasite in our brains - its effects are probably very subtle but I would be quite confident that we will find evidence of its impact in most species".